The invention relates to a method for calendering a board web.
A Yankee cylinder is generally used in the manufacture of sufficiently stiff board grades which are suitable for packages, for example, for biscuit/cookie packages, cosmetic packages, etc. and one side of which is additionally required to have good surface properties. After the treatment with a Yankee cylinder, the board surface which has been against the Yankee cylinder is subjected to surface treatment. After the surface treatment, the board web is further subjected to final calendering, when needed. The web treated with a Yankee cylinder is given good surface properties, good bulk and stiffness and low shrinkage at the edges.
The most important quality properties of board produced by a Yankee cylinder before coating are in a range: bulk 1.4-1.6 dm3′/kg, Bendtsen roughness 50-250 ml/min and PPS-s10 roughness 3.5-7.5 μm.
One problem with the use of a Yankee cylinder is its runnability. The Yankee cylinder can be run only in a relatively narrow operating window. The web must be sufficiently moist when it arrives at the Yankee cylinder in order that it should properly adhere to the hot and smooth outer surface of the shell of the Yankee cylinder by the action of adhesion. On the other hand, the web must not be too moist when it arrives at the Yankee cylinder in order that it shall have time to dry sufficiently on the Yankee cylinder. If the web does not have time to dry sufficiently on the surface of the Yankee cylinder, it cannot be detached from the surface of the cylinder at the trailing end of the cylinder. Separation of the web from the hot outer surface of the Yankee cylinder takes place by means of a doctor. The above-mentioned runnability problems have limited the running speed of the Yankee cylinder with board grades of this kind to a range of below about 600 m/min. Typically, the running speeds of the Yankee cylinder are in a range of about 200-400 m/min. Moreover, the shell of the Yankee cylinder, the diameter of which may be as large as 7 m, shall meet strict requirements in respect of deformation, thermal conductivity, wear and corrosion, with the result that the Yankee cylinder will be relatively expensive. In addition, an impingement device is normally used in connection with the Yankee cylinder.
The above-mentioned board grades can also be produced without a Yankee cylinder, in which connection the web can be provided with desired surface properties by means of a wet stack calender. The wet stack calender is formed of a multinip hard nip calender, but the calendering process totally differs from conventional hard nip calendering. The wet stack calender makes use of moisture gradients. The web is dried before the wet stack calender such that its moisture content is only about 1-2%. On the wet stack calender, water boxes are used in connection with 1-3 rolls for forming a film of water onto the outer surface of the roll shell before a nip. This water film is pressed onto the surface of the web in the nip. The relatively thick web is moistened only from the surface thereof, in which connection, by the action of simultaneous pressure, the web is calendered more on the surface as compared with the over-dried interior of the web. This kind of calendering results in a good smoothness to bulk ratio, i.e. good smoothness is obtained, however, without losing too much bulk. Surface treatment and possibly final calendering of the web are carried out after the wet stack calendering.
Runnability problems are also associated with the wet stack calendering. If the pressure distribution in the nips provided with water boxes is not sufficiently uniform, water can pass through the nip, forming a pocket of water underneath the web. This causes web breaks at the next nip. Since bulk is a critical factor with board grades, it must be possible to operate the calender with an optimal nip pressure required by each board grade, which pressure is sufficiently uniform in the entire area of the nip allowing the use of water boxes. The wet stack calender is designed so that the number of rolls can be varied therein and deflection-compensated rolls are placed such that a sufficiently uniform nip pressure is achieved in the nips provided with water boxes. In the wet stack calender, wrinkles are also readily formed in the web, in particular in a web having a low basis weight.
Metso Paper, Inc.'s U.S. Pat. No. 5,938,895 discloses one wet stack calender with a water box positioned in connection with a profiling nip formed by a deflection-compensated roll placed in the middle of the calender. U.S. Pat. No. 5,522,312 discloses a wet stack calender in which a metering device is used in connection with a water box for controlling the thickness and uniformity of a water film applied to a calender roll. The water is transferred from the calender roll to the web at a nip. U.S. Pat. No. 5,607,553 discloses a wet stack calender in which the water boxes have been replaced by water spray devices for spraying water in the form of droplets to a reversing roll of the calender. The water is transferred from the reversing roll to the web at a nip.
WO 99/67462 discloses a method for calendering paper or board when manufacturing coated grades of paper or board in two stages. In the first stage, i.e. the pre-calendering stage, the uncoated web is calendered with a shoe calender having a shoe length of at least 50 mm after which the web is coated. In the second stage the coated web is calendered with a calender having a nip length of 50 mm at the most. The maximum pressure in the pre-calendering nip is kept at 0 to 15 Mpa, preferably at 4 to 12 Mpa. The web is pre-calendered at a moisture and temperature where at least the glass transition temperature of the material forming the surface part of the web has been reached. The web may be pre-treated e.g. by steaming and/or wetting with water or a combination of pre-wetting and the use of a heated backing roll in the shoe calender can be used in order to reach the glass transition temperature. The dwell time of the web in the calender is 3 to 40 ms.
WO 96/26809 discloses a coated paperboard for formed articles, e.g. liquid packaging board or white top liner. The paperboard consists of a fibermatrix in one, two or more layers and a coating and has adequate surface gloss for each specific type of formed articles. The paperboard has been calendered only after it has been coated with a heated calender having a soft extended nip. The specification mentions that the calender temperature is typically in the range of 140-250° C., but even higher temperatures are possible. The length of the nip in the so called supersoft calender which can be used in calendering the coated web is said to be 40 to 60 mm. Although the main idea in this document is to calender only the coated web, i.e. not the uncoated web it is said that it is also possible to calender the uncoated web in addition to the calendering of the coated web.
WO 00/03087 discloses a method for manufacturing surface-treated printing paper. The web is first dried down in the drying section typically to a moisture of 2 to 4%, and it is wetted again typically to a moisture of 8 to 12% before surface treatment, e.g. calendering in a multi-nip calender. The wetting device is placed before the calender so that the absorption time of the water is 0.2 to 2.0 s before the web reaches the calender. The amount of water used in the method is 0.1 to 10 g/m2 and the droplet size in the spray-moistener is 10 to 100μ.
U.S. Pat. No. 5,836,242 discloses a calendering system in a papermaking or board manufacturing process. The system comprises at least one press nip and an endless calender belt comprising a core and a compressible, elastic material bonded to the core. The belt comprises at least two layers having different hardnesses. The web side has a first hardness in the range of 75 to 91 Shore A and the layer on the press side has a second higher hardness. The average pressure used in this calender is higher than 4 MPa.
Because of the above-mentioned limitations, in the case of the above-mentioned board grades it would be desirable to replace the Yankee cylinder and the wet stack calender with some new improved arrangement. As one new arrangement, trials have been carried out with the long nip calender known in itself, but so far the results have not been good enough.
By a long nip calender is meant a calender in which a nip is formed between a heatable steel roll and a belt. In the long nip calender, the nip pressure can be adjustable in the cross direction (CD) of the machine, in which connection it is possible to profile, for example, the caliper of paper. The long nip calender can be formed of a belt calender in which a belt is passed as guided by auxiliary rolls around one of the nip rolls, i.e. the roll operating as a backing roll for a thermo roll. Thus, a long nip is formed between a thermo roll and the other nip roll loading the belt. The most common long nip calender is a shoe calender in which a belt is arranged to run around a stationary support structure and in which the belt is loaded against a thermo roll by means of a loading shoe positioned inside the loop of the belt and supported on the support structure. The long nip is formed in the shoe calender between the thermo roll and the shoe loading the belt. Thus, the length of the nip is determined by the loading shoe of the shoe calender. The method according to the invention is primarily suitable for use in connection with the above-mentioned shoe calender.
In the prior art shoe calenders which are in use, the length of the nip is typically in a range of 50-70 mm, i.e. the dwell time of the web in the nip is considerably less than 10 ms. The surface temperature of the thermo roll serving as the backing roll of the shoe roll is in a range of 80-200° C. and the maximum pressure of the nip is in a range of 5-10 MPa. The hardness of the calender belt of the shoe calender is in a range of 80-100 ShA. Longer nips of about 270 mm have been used in presses based on the shoe roll.